1. Field of the Invention
The present invention relates generally to the field of the measurement of silicon wafers used in the production of semiconductors, and particularly to the measurement of the thickness of thin wafers, the flatness and localized shape of thin wafers and the depth of trenches etched thereon.
2. Description of the Prior Art
When making semiconductors, manufacturers start with blank silicon wafers. Many processes are performed on them before the completed semiconductors are complete and a large number of these processes involve placing images of a photo mask on the wafer. The various images must overlay each other with great accuracy. As the size of the features on the wafer shrink, the accuracy with which each layer must overlay increases.
The size of features being placed on wafers now is at such a tight level that even the shape of the wafer can effect the quality of the photo processes. Imagine a wafer with the shape of a potato chip. The wafer is so warped that the masks can never be aligned because the optical equipment cannot focus on the entire surface at once. This type of problem is costly to semiconductor manufacturers due to reduced yields. An instrument that accurately and reliably measures the flatness and thickness of a wafer would help these manufacturers improve their process and produce better ICs with greater yield.
The present technique that is most popular for measuring the thickness of a wafer is called a capacitance test. The wafer is placed between two electrodes and the material in between causes a change in capacitance. The change in capacitance is a direct measure of the amount and type of material between the electrodes.
For many years this technique has worked reliably. The limitations and shortcomings of this technique have only recently become significant as the accuracy of the required measurement increases and the wafers have become significantly thinner.
One of the shortcomings of this technique is that detailed information is needed about the properties of the wafer material, such as its relative permittivity. This can be problematic if the wafer has multiple materials or is bumped with solder bumps.
Another shortcoming is that the number of locations where the thickness can be measured is small, due to the relatively large size of the capacitance sensors. Typically the number of locations is about 10 for a 4-inch wafer and about 30 for a 12-inch wafer. A manufacturer would ideally want to know detailed height and thickness information over the entire wafer, not just a small number of locations. Additionally, the accuracy of the measurements is a question as the measurement is essentially an average thickness over the area of the capacitance sensor.
The area of the capacitance sensor is typically about ½ inch in diameter. More importantly, however, is that the resolution of the capacitance sensors is no longer fine enough to satisfy the increasingly tight requirements of the manufacturers. With late generation wafers having a thickness of between 800 and 200 microns, and expectations that future generations will be as thin as 40 microns, the measurement accuracy needs to be 0.1 micron or smaller.
Related to the problem of measurement of the thickness of thin wafers is the problem of measuring trench depth on wafers. When processing semiconductor wafers into devices, such as integrated circuits, micro-electro-mechanical systems (MEMS), and integrated photonic devices, manufacturers perform many processes, some of which include etching trenches into the wafer. For many of these devices, the depth of the etched trench is critical to the proper performance of the finished device, and the manufacturers typically desire to measure its depth. However, current methods of measuring the trench are severely limited.
MEMS products typically contain three-dimensional structures with regions of deep, narrow trenches with near-vertical sidewalls. A typical example is a trench etched 5 microns wide by 100 microns deep. MEMS devices with these characteristics include sensors, actuators, and RF devices such as inductors and comb switches. All of these devices characteristically require deep vertical etching processes to separate moving mechanical parts, and finger-like features are very common.
Manufacturers of MEMS devices do not currently have an accurate and inexpensive method to non-destructively measure the depth of etched high aspect ratio trenches. They need to have precise control over etch depth to produce a working device, and the measurement of etch depth is very important for process development and control. Current metrology technology cannot measure the depth of high aspect ratio trenches with speed and accuracy. Thus, the development of a non-contact metrology instrument that quickly and accurately measures the etched depth of high aspect ratio trenches, such as those formed by narrow finger-like structures, would greatly benefit MEMS manufacturers in process development and control.
Integrated circuits often require deep trenches etched in the wafer to electrically isolate neighboring circuit devices, such as transistors. Space on the wafer is always an important consideration, and yet the trench must be deep enough to provide the required isolation. Thus, the aspect ratio of these etched trenches is increasing with improvements in technology. Currently, these trenches can be one micron or smaller wide and six microns or more deep. In addition, these trenches typically have rounded or rough bottoms that absorb any incident light. Manufacturers need to measure the depth of these trenches for process control and characterization. Currently, the only method to measure these trenches involves destructively cutting the wafer.
Integrated photonics devices are typically fabricated on materials other than silicon, or in layers of materials “grown” on top of silicon. Examples are SiC, InP, GaAIAs, and silicon nitride. These devices are etched structures to form waveguides, lasers, and other photonic devices. The shape parameters of the etched structures are very important to the performance of photonic devices. The current invention relates to the measurement of deep trenches in a wide variety of materials, such as the above examples as well as the thickness of the wafer.
Because of the very steep sidewalls inherent in such trench structures, profiling instruments that use a stylus or other method of contact cannot accommodate an aspect ratio or lateral dimension of this nature. For example, atomic force microscopes (AFM) and stylus profilers are not suitable because even if the tip could penetrate the trench, it would not be able to follow the side wall, and the tip would break when exiting the trench.
Standard non-contact optical instruments for measuring surface height are confocal microscopes, white light interferometers, phase shift interferometers, and triangulation techniques. All of these optical techniques involve some manner of illuminating the trench and analyzing the reflected light. However, the steep walls of the trench prevent much of the light from reaching the bottom of the trench. In addition, some etched trenches may have rounded or rough bottoms. The light that enters these trenches might be completely absorbed. If there is no light returned, then no method of analysis can possibly determine the depth of the trench. Aside from these fundamental problems, each of the listed non-contact methods has problems unique to that particular method.
Standard confocal microscopes fail because they confuse the signal from the top of the trench with the signal from the bottom when the trench is too narrow. When the width of the trench approaches the size of the source pinhole, as much or more light will be detected when the focus is on top of the trench as when it is at the bottom. Thus, a confusing signal is generated even when the bottom of the trench is far away from the focal plane. Confocal microscopes are also very slow since they require scanning the measurement sample axially to find the plane of best focus.
White light interferometers have similar difficulties in that they are slow and must scan axially. In addition, the fringe signal is weak due to the light scattered from the walls and the top. Phase shift interferometers fail outright because phase unwrapping fails to detect steep sidewalls. Finally, triangulation techniques can only succeed if precise control of the direction of the incident beam relative to the direction of the trench is maintained so that the light can get into the trench from the side. This constraint makes such an instrument infeasible.
All of the prior art methods described above have in common the fact that they attempt to measure the trenches from above, that is, the optical beam or mechanical stylus approaches the trench from the same side as the surface that was etched.